1. Field of the Invention
This invention relates to tonometers, devices used for measuring the intraocular pressure (IOP) of an eye, and more specifically to a non-contact type of tonometer which utilizes an ultrasonic beam to apply a pressure on the eye to cause applanation or indentation and an optical or ultrasonic beam to measure the applanation or indentation.
2. Description of the Prior Art
Glaucoma affects as much as 2% of the population over 40 and is a leading cause of blindness. It damages the eye through increased IOP. Unfortunately, current devices for measuring of the IOP, called tonometers, fall far short of the ideal instrument in almost every respect, in spite of the innovative contributions by many workers since the eighteen hundreds. Over time, various mechanical devices, electronic devices, and most importantly, devices which do not touch the eye have been developed. In spite of intense activity over more than one hundred years, the ideal device has not been previously developed. An ideal tonometer would have several characteristics. First, it should not touch the eye during the measurement; that is, it should be a non-contact tonometer. This avoids the possible transmission of disease between patients, the possibility of corneal scarring, and the risks for adverse patient reaction to the topical anesthetic required for contact tonometry. Second, it should be comfortable to the patient and quiet in operation so as to not startle the patient. Third, it should have an accuracy of at least .+-.0.5 mmHg over the entire range of IOP's from 10 mmHg to 60 mmHg. Fourth, in use, the results should be insensitive to the technique of the clinician and the measurements should be easy to perform so that tests may be readily performed by clinical assistants. Fifth, the device should be of low cost. Sixth, an ideal device should be amenable to home use. Seventh, the device should be capable of use on patients whether they are supline or erect. Eighth, it should not require periodic recalibration. And ninth, it should be made to be small and hand-held so as to be useful for general practitioners for screening and for use in emergency rooms.
Maklakov developed the impression tonometer in 1885. In this device, a plunger of a known weight is allowed to rest against the eye with the patient in a supline position. The area of contact is determined with a dye, and the weight divided by the area of contact gives the pressure. An indentation tonometer was subsequently developed by Schiotz. In this device, a circular foot-plate rests on the eye and a central plunger of fixed weight slides down against the eye. The depth of indentation is indicated by a mechanical lever system and the indentation bears a general inverse relationship to the IOP. However, due to the complex effects of substantial fluid displacement, the IOP must be determined using a complex table. Another disadvantage is that the corneal rigidity and the corneal radius both affect the measurement. Yet another disadvantage of this tonometer is that it is of delicate mechanical construction, requiring great care in use. In spite of the disadvantages of the Schiotz and Maklakov tonometers, they are both used around the world today. A tonometer having an accuracy greater than that which can be achieved using the Schiotz tonometer was developed in the 1950's by Goldmann (U.S. Pat. No. 3,070,997). In this device, a small biprism is pressed against the cornea. The cornea is prepared by applying a topical anesthetic and a dye which is illuminated by a slit lamp. The image of the glowing tear film around the edge of the prism is split by the biprism such that when just enough pressure is applied to applanate (flatten) the cornea to the diameter of the prism, the half-images of the glowing ring become perfectly aligned. Great skill is required in order to obtain accurate measurements and the disturbances of the intraocular pulse must be averaged out visually. The effects of the attractive force of the surface tension of the tears and the repulsive force due to the bending of the cornea both affect the measurements. For this reason, the diameter of the prism is selected to compensate for these forces on the typical patient as much as possible. In spite of this, the Goldmann tonometer is of questionable accuracy in practice due to problems related to calibration and clinical technique. Also, being a contact tonometer, it can damage the cornea and communicate disease between patients. Finally, it is not easily used by clinical assistants. For the above reasons, it fails to meet the ideal tonometer criteria in most ways. It is, however, sufficiently accurate in the hands of a skilled clinician to be clinically useful for both treatment and screening. In another contact tonometer, developed by Makay and Marg, a cylindrical and hollow thick-walled tube is contacted to the eye (U.S. Pat. Nos. 3,049,001, 3,150,520, and 3,150,521). A central plunger is then used to measure the restoring force of the eye. This device has also been implemented in a hand-held device about the size of a fountain pen of large bore (U.S. Pat. No. 4,747,296). Measurements with this device are very technique dependent and this tonometer is not widely held to be clinically useful. In another attempt to develop a better tonometer, in the 1960's, one Grolman developed the air-puff or fluid discharge tonometer (U.S. Pat. No. 3,538,754). In this tonometer, a short duration puff of air is directed toward the eye. The pressure of this discharge increases with time. When the instantaneous pressure of the air stream at the surface of the eye is equal to the IOP, the cornea is flattened. An optical system detects the moment of applanation, and, by synchronization with the air-puff initiation, can indicate the IOP. This device has not been well accepted by patients since both the noise of generating the air-puff and the sensation of the air against the cornea are objectional. Furthermore, the device is of little clinical value above 30 mmHg due to lack of accuracy. Some versions of the device are so sensitive to clinical technique and alignment that many clinicians cannot obtain results with them at all. Various other devices have been patented but not utilized commercially. Most follow the lines of the various devices described above. One additional type is the vibration tonometer, first patented in the 1960's (U.S. Pat. Nos. 3,192,765 and 3,882,718). In this device, it is proposed that the response of the eye to a vibrational excitation will be a measure of the IOP. The proposed exciters include very low-frequency sound and mechanical plungers. However, it is likely that the vibrational frequencies of the eye are affected by many factors not related to the IOP. It is, in fact, expected that the actual resonance spectrum of the eye would be dictated more by the connective tissue than by the IOP. All of these factors may be the reason why no commercial use of the vibration tonometer has been disclosed even though its development has been attempted. In addition, the vibration is likely to be very uncomfortable to the patient. Applicant is unaware of any commercial application of this concept in spite of attempts to build a working device. In summary, no known tonometer meets all of the desired criteria, and none meet the actual clinically preferred level of accuracy and independence of measurements from the clinical technique used. Thus, it is desired to provide an improved non-contact tonometer which meets the above indicated criteria and avoids the shortcomings of the prior art devices.